usb: musb: blackfin: include prefetch head file

After the prefetch/list.h restructure, drivers need to explicitly include
linux/prefetch.h in order to use the prefetch() function.  Otherwise, the
current driver fails to build:
drivers/usb/musb/blackfin.c: In function 'musb_write_fifo':
drivers/usb/musb/blackfin.c:43: error: implicit declaration of function
'prefetch'

Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>